The present invention relates to condensing furnaces, and more particularly to a condensing heat exchanger in a gas fired condensing furnace for transferring sensible and latent heat energy from a heating fluid to a flow of air to be heated.
Due to the cost and shortage of natural gas, attempts have been made to design and construct more efficient gas fired hot air furnaces. One successful method in maximizing the heat energy transferred from a heating fluid to air to be heated is to transfer as much latent heat as possible from the water vapor in the heating fluid to the air. Thus, increases in furnace heating efficiency have been accomplished by purposely lowering the heating fluid's temperature and condensing as much of the water vapor as possible to transfer its latent heat to the air. This is generally accomplished by including a condensing heat exchanger with the primary heat exchanger and passing cool air to be heated initially over the condensing heat exchanger. In these types of condensing furnaces, furnace heating efficiencies of 90% or more are not uncommon.
In general, most of these condensing heat exchangers take the form of a shell and tube, fin tube, or plate fin arrangement for further reducing the temperature of the heating fluid. Although these arrangements have been somewhat successful, due to their structure and material requirements they are relatively costly to produce and generally result in too large of a pressure drop of the fluid flowing thereacross. One method of overcoming the large pressure drop is to provide a larger cabinet within which the furnace assembly is enclosed or to provide a blower of increased capacity. However, either one of these two solutions serve only to increase further the size and cost of the condensing furnace.
Further, arrangement within prior art condensing furnaces of the burner device, primary heat exchanger, condensing heat exchanger, and blower are such that additional piping is usually necessary to deliver the flow of heating fluid from the primary heat exchanger to the condensing heat exchanger so that condensate formed therein does not flow to the burner device or blower assembly. Since the condensate is generally acidic, its corrosive effects on the burner device or blower assembly would be highly undesirable.
In reference to the design of primary heat exchangers, one design is known as the clamshell type since it is constructed from two separate sheets, usually of metal, each of which has the design stamped therein and are then secured together to form the heat exchanger. Although using two separate sheets has been generally satisfactory in producing the primary heat exchanger, which would handle only gaseous flue products, its use in manufacturing a condensing heat exchanger is not as desirable since the liquid condensate tends to leak therefrom.